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Inflatable Moth Butt Featherdusters

THIS IS NOT A PENIS. Chionarctia nivea; a male tiger moth from Japan. Original image from Acleris.

A photo is making the Internet rounds this week of a moth with a surprising... well, just look at it. What the heck is going on?

What's popping out of that male moth is NOT a penis. It's almost as interesting as a penis, though–it's coremata. That's a Greek word for "feather duster;" it's an eversible scent gland of some male moths. They are especially common in tiger moths and wooly bear moths.

As little animals that live widely dispersed, finding a partner of the proper species to reproduce with is a problem. How do moths arrange a hookup? Sexual signals via chemical pheromones are a common solution. In both males and females, glands inside the body produce the signaling chemicals, and are everted outside the body to release the scent. The male glands can be fairly spectacular. From one paper:

"coremata are huge, pneumatically eversible organs composed of two pairs of tubes, up to 37 mm long, each covered by ca. 3000 scent hairs (scales)"

Coremata can be as long as, or longer, than the moth's entire body; they range in shape from little butt pom-poms to large tubes. The hairs all over the gland help waft the chemicals (Axe™ Body Spray for Moths?) around to entice a female; often males will flutter their wings to help spread the aroma. It's also common for males to stroke the head of the female with their feathery butt appendages. SEXY.

These organs are inflated with either air or blood; in the photo above a recently-deceased moth was inflated with air blown through a pipette into the moth's body cavity. A living male moth would inflate his coremata by pumping his abdomen rapidly to draw in air.

Upon contact with a receptive female the male's coremata deflate, and his penis inflates. Then the male still has to manage to "Insert Tab A into Slot B," if you will, and convince the female to let him stay there long enough to transfer a sperm packet.

“The Phalloblaster inflates the genitalia with a stream of pressurized alcohol to create the same shape as when the insect was alive.
The alcohol dehydrates and hardens the structure, so that once the process is over the genitalia remain inflated rather like miniature balloons. It makes them easier to study.”